The inguinal administration of a co-carcinogen, TPA, in combination with normal guinea pig spleen cells or alone into normal guinea pigs chronically infected with the latent guinea pig herpes virus has resulted in 12 animals dying from a fulminating leukemia. Seven of the leukemias have occurred in male animals. Histologically, the new leukemias have been defined as undifferentiated lymphomas, B cell in origin, and mimic the histological pattern seen in Burkitt's lymphoma. These leukemias were found to be histologically and immunologically identical to the L2C-EN (Gross) leukemia in pigs, a disease of female origin. A specific number of chronically infected EBV and HVS animals when adiministered procarbazine and TPA in combination died from leukemia. Histologically, these leukemias were indentical to the leukemias induced with TPA alone.